The present invention relates to sintered metal filters, and, more particularly to a method for making such filters for removing efficiently extremely small particles from a stream of gas passing therethrough.
As is well known, high efficiency porous sintered metal filters are widely utilized in the electronic and other industries wherein an extremely clean environment is necessary in order to avoid contamination of sensitive products. For example in the electronics industry, inline filters are frequently used to treat the process gases to avoid introduction of any particulate matter in the manufacturing process for semiconductors and thereby contamination of the electronic products.
The electronic industry has recognized the advantages in use of sintered metal filters to achieve highly efficient removal of small particles. Illustrative of structures for this purpose are those indicated in Davis U.S. Pat. No. 5,114,447 granted May 19, 1992 and Zeller U.S. Pat. No. 5,487,771 granted Jan. 31, 1996.
Although the filters shown in these patents are effective to remove small particles in the gas stream, the relatively large internal volume of the filter chambers which they require relative to the actual volume occupied by the filter element provides dead space for gas entrapment and moisture buildup. The special housing designs to receive such filters and provide sufficient filter area often require special closures and multiple welds, and these must be helium tested or pressure tested. Moreover, multipart housings increase the costs and time for manufacture.
The electronics industry is desirous of inline filters which may be readily installed and quickly purged and which will achieve a high efficiency rate of removal of 99.9999999% determined at a most penetrating particle size i.e. log reduction value (9LRV), at the rated flow. The test methodology for evaluating 9 log removal rating is described in Rubow, K. L., D. S. Prause and M. R. Eisenmann, "A Low Pressure Drop Sintered Metal Filter for Ultra-High Purity Gas Systems", Proceedings of the 43rd Annual Technical Meeting of the Institute of Environmental Sciences, Los Angeles, Calif. May 6-8, 1997, and Rubow, K. L. and C. B. Davis, "Particle Penetration Characteristics of Porous Metal Media for High Purity Gas Filtration", Proceedings of the 37th Annual Technical Meeting of the Institute of Environmental Sciences, PP834-840, 1991.
The industry also wishes to achieve relatively high flow rates with relatively low pressure differential losses across the filter.
It is an object of the present invention to provide a novel method for making a porous sintered metal filter element of cup-shaped configuration which provides ultra-high efficiency in minute particle removal (9LRV) and exhibits low pressure differential losses.
It is also an object to provide such a method for making such an ultra-high efficiency filter element which may be conducted readily and at relatively low cost.
Another object is to provide such a cup-shaped ultra-high efficiency filter element which provides a relatively large area for flow of gas therethrough and exhibits relatively low pressure drop.
A further object is to provide such a filter element which exhibits a 9 log efficiency rate of removal of the most penetrating particles at a stated maximum flow rate.